add ClideMarkdown, ClideCodeBlock, ClideSvgView primitives

Three reusable rendering widgets in lib/widgets/:

- ClideMarkdown: walks markdown AST (GFM tables, task lists,
  fenced code), renders headings, lists, blockquotes, tables,
  inline bold/italic/code/links. Delegates code blocks to
  ClideCodeBlock.

- ClideCodeBlock: syntax-highlighted code via TreeSitterService.
  Async highlight, byte-to-char offset mapping, theme-aware
  colors from SurfaceTokens.

- ClideSvgView: wraps jovial_svg with asset/string constructors
  and optional sizing.

Retrofitted: decision detail body renders as markdown instead of
raw text. Markdown viewer uses ClideMarkdown. Welcome logo uses
ClideSvgView.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

lib/builtin/decisions/src/decision_detail_view.dart

@@ -108,15 +108,7 @@ class _DecisionDetailViewState extends State<DecisionDetailView> {
           ),
           if (body != null && body.isNotEmpty) ...[
             const SizedBox(height: 12),
-            Container(
-              padding: const EdgeInsets.all(12),
-              decoration: BoxDecoration(
-                color: tokens.panelBackground,
-                borderRadius: BorderRadius.circular(4),
-                border: Border.all(color: tokens.panelBorder),
-              ),
-              child: ClideText(body, fontSize: 13, fontFamily: clideMonoFamily),
-            ),
+            ClideMarkdown(body),
           ],
           if (refs.isNotEmpty) ...[
             const SizedBox(height: 16),

lib/builtin/markdown/src/markdown_viewer.dart

@@ -78,16 +78,12 @@ class _MarkdownViewerState extends State<MarkdownViewer> {
         child: ClideText('Open a .md file to preview it here.', muted: true),
       );
     }
-    final tokens = ClideTheme.of(context).surface;
     return ClidePaneChrome(
       title: _path ?? 'viewer',
       subtitle: '${_content!.split('\n').length} lines',
       child: SingleChildScrollView(
         padding: const EdgeInsets.all(12),
-        child: Text(
-          _content!,
-          style: TextStyle(color: tokens.globalForeground, fontSize: 13, fontFamily: clideMonoFamily, fontFamilyFallback: clideMonoFamilyFallback),
-        ),
+        child: ClideMarkdown(_content!),
       ),
     );
   }

lib/builtin/welcome/src/welcome_view.dart

@@ -2,9 +2,7 @@ import 'dart:async';
 
 import 'package:clide/kernel/kernel.dart';
 import 'package:clide/widgets/widgets.dart';
-import 'package:flutter/services.dart' show rootBundle;
 import 'package:flutter/widgets.dart';
-import 'package:jovial_svg/jovial_svg.dart';
 
 class WelcomeView extends StatelessWidget {
   const WelcomeView({super.key});
@@ -56,13 +54,7 @@ class _Header extends StatelessWidget {
     return Row(
       crossAxisAlignment: CrossAxisAlignment.center,
       children: [
-        SizedBox(
-          width: 144,
-          height: 144,
-          child: ScalableImageWidget.fromSISource(
-            si: ScalableImageSource.fromSvg(rootBundle, 'assets/logo/logo.svg'),
-          ),
-        ),
+        const ClideSvgView.asset('assets/logo/logo.svg', width: 144, height: 144),
         const SizedBox(width: 24),
         Column(
           crossAxisAlignment: CrossAxisAlignment.start,